1. Field of the Invention
This invention relates generally to laser communications systems and more particularly to a pulsed digital laser generator therefor.
2. Description of the Prior Art
The processing steps of a digital communications system are from the information source through the transmitter, receiver modems and ultimately to the information sink. The key feature of a digital communications system is that it only sends a set of finite messages in contrast to an analog communications system which can send an infinite set of messages. The objective at the receiver end of the digital system is not to reproduce a waveform with precision, but on the other hand, it is the objective to determine from a noise perturbed signal which of the finite waveforms had been sent by the transmitter. These properties make it particularly useful for satellites, networks, telephony and other appliations where computers need to communicate with other computers.
Laser communications systems are generally broken down into a generator to produce the carrier wave, a modulator to impress information on the carrier according to an input signal, a transmission medium, a receiver and demodulator. In digital systems, use is made of pulse code modulation (PCM) in which one bit of digital information in the input signal is represented by the carrier being in one of two binary states at a particular time. The efficiency of such a system depends on how much information the laser beam can be made to carry in the specific time period, how much of the energy of the carrier is wasted in effecting the modulation, and how well the difference between the two carrier states is observed in transmission.
In fulfillment of these objectives, an impressive assortment of signal processing techniques has been proposed. One such technique comprises the apparatus disclosed in U.S. Pat. No. 3,617,710, entitled, "Multiplexed Digital Laser Generator", which issued to Thomas E. Honeycutt, one of the present inventors, on Nov. 2, 1971. Disclosed there is a laser transmission system including a generator which transmits several channels of digital information simultaneously. This system, however, utilizes a continuous wave (CW) laser which is wasteful of energy and severely limits the number of bits that may be concurrently transmitted. The number of bits which may be transmitted at any point in time is governed by the bandwidth of the laser gain curve above the threshold for inhomogeneously broadened laser lines. In CW lasers of reasonable length, only one longitudinal mode can be made to lase at a time; however, several frequencies can still lase at once by exciting one or more transverse modes. This tends to promote mode competition and makes it difficult to control the frequencies which are being emitted. The teachings of this patent, furthermore, are intended to be specifically incorporated herein by reference.
Accordingly, it is an object of the present invention to provide an improvement in laser communications systems.
It is another object to provide an improvement in digital laser communications systems.
It is still another object of the present invention to provide a pulsed digital multiplexed laser communications system.
It is a further object of the present invention to provide a pulsed digital multiplexed laser communications system which overcomes the limitations imposed by mode competition.
It is yet a further object of the invention to provide a pulsed digital multiplexed laser communications system which operates at an increased laser gain with an additional increase in efficiency.